With regard to a receiver circuit of terrestrial digital broadcasting, there is a known method called diversity combining (which is also referred to as diversity reception) which combines signals received using a plurality of antennas for the purpose of reception performance improvement. A combination ratio of the diversity combining is decided on the basis of quality information (for example, carrier-to-noise ratio (CN)) that is calculated for each antenna and indicates quality of a reception signal.
The method for calculating quality information can be a method using a modulation error ratio (MER). The MER is calculated on the basis of a distance of a reception signal point from an ideal reception signal point. The reception signal point is obtained by demodulating a data signal (modulated signal) in an orthogonal frequency division multiplexing (OFDM) carrier and plotting it on an I-Q plane. The data signal is modulated by a modulation method, such as quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (QAM) or 64QAM, for example. The distance between the reception signal point and the ideal reception signal point is caused by noise. As the distance is larger, the quality is worse (CN is lower). As the distance is smaller, the quality is better (CN is higher).
See, for example, Japanese Laid-open Patent Publication No. 2012-109993.
However, when a distance between a reception signal point and an ideal reception signal point corresponding to the reception signal point becomes larger, it is possible that another ideal reception signal point that is closer to the reception signal point is used for calculating the distance. In that case, the calculated distance is smaller than reality, and the accurate quality information is unable to be obtained. As the modulation method is less tolerant to noise, the distance is more likely to be calculated erroneously.
To solve this, one can conceive of calculating quality information, using a control signal (auxiliary channel (AC), transmission and multiplexing configuration control (TMCC), etc.) which is modulated by a modulation method that is more tolerant to noise than a modulation method of a data signal is. However, unlike signals combined by diversity combining, transmission channel correction (e.g., equalization) for reducing inter-carrier interference is not performed on control signals, such as AC and TMCC. The quality information calculated from such control signals is inaccurate, in some cases.
As described above, when the quality information used in calculating a combination ratio is inaccurate, diversity combining is not performed accurately.